To the Editor:
“Twenty-Year Follow-up of the Breast Cancers Diagnosed During the Breast Cancer Detection Demonstration Project” in the May/June 1997 issue1 was an excellent paper. In Table 5 the authors present the adjusted and observed survival rates of patients with breast cancer. As a clinician, I would find it helpful to have an explanation of the terms “adjusted survival” and “observed survival.”
Dr. Lawrence's question about the definitions of “adjusted survival” and “observed survival” is important.
As mentioned in the “Methods” section of the article, we considered various measures of survival in our analysis of the Breast Cancer Detection Demonstration Project (BCDDP). Adjusted survival appeared to be the most appropriate measurement of outcome for this study. In this analysis, information on the cause of death was taken from the death certificate, and “adjusted survival” counted only deaths from breast cancer as “events.” Breast cancer was considered the cause of death whether it was a primary or a contributing cause.
Thus, in the survival analysis, women who had breast cancer but died of other causes contributed to follow-up until the time of death, but their deaths were not counted as events. Evaluating only breast cancer death as the outcome provided a way for us to “adjust” for other causes of death in this analysis.
In contrast, the observed survival presented in Table 5 counts any death regardless of cause as an event.
Because one goal of our analysis was to make comparisons among women diagnosed with breast cancer at different ages, comparing observed survival by age is problematic because death rates from all causes increase as women get older. However, since some argue that the cause of death is less important than whether the patient is alive or dead, we presented the observed survival for comparison.
We also wanted the analysis to consider the question “to what extent did the diagnosis of breast cancer affect the survival of women participating in the BCDDP?” To answer this question we would need to know the expected survival rates for these women if they had not been diagnosed with breast cancer. As a substitute for this information, studies such as ours often use death rates from a national population. These rates would be the basis for estimating relative survival, or the ratio of the observed survival rate to the expected survival rate for the cohort.
The women who volunteered for the BCDDP were not a representative sample of the US population, however, and other analyses have shown that their age-specific death rate for all causes is much lower than national rates.1 As described in the “Methods” section, the adjusted breast cancer survival and the relative survival from the population-based Surveillance, Epidemiology and End Results (SEER) program are quite similar (57% and 53%, respectively). Because we did not have appropriate external death rates to apply to the BCDDP population to use in the calculation of relative survival, adjusted breast cancer survival rates provided the closest measure to address this question in the BCDDP.